!!! - be sure your ad-block is OFF. AdCash panel may work incorrectly with ad-blocks.

AdCash API doesn't support auto-import. If you want to import popunder ads you may easily make it manually.
You need to know two things: popunder link URL and zone_id.
Go to AdCash panel, find your website in 'Zones' section, see the screenshot below:

You will see a list of your websites and ad zones, scroll down to your website and find your popunder zone:

Click on '+' and open zone properties:

!!! Select https protocol
Copy your link URL and ID to AdSpyglass:

Scroll down the page and click on Save button: